1. Field of the Invention
The invention relates to optical disk drives, and more particularly to signal calibration of optical disk drives.
2. Description of the Related Art
A tracking error (TE) signal is one of the important servo control signals of an optical disk drive and corresponds to controlling the horizontal motion of a pickup head. Prior to using the tracking error signal as the source for controlling motion of the pickup head, the tracking error signal must be appropriately amplified to a target signal amplitude and compensated to a target signal level. This process is referred to as a calibration process. The calibration of a tracking error signal is important in optical disk drives. If the calibration does not precisely amplify and compensate the tracking error signal, the horizontal motion control of the pickup head is misdirected, thus, data retrieval from the optical disk drive is hindered.
The tracking error signal is often affected by a run-out phenomenon. FIG. 1A is a schematic diagram showing a run-out phenomenon. When an optical disk is inserted into an optical disk drive, the physical center 172 of the optical disk does not often coincide with the rotation center 160 of a turntable. Thus, when a spindle motor spins the turntable, the optical disk shifts left and right. The left-shifted optical disk 162 has a center located at position 172, and the right-shifted optical disk 164 has a center located at position 174. If the distance between the physical center of the optical disk and the rotation center of the turn table is d, the maximum shift distance of the optical disk, referred to as a run-out distance, is 2d, as shown in FIG. 1A.
When the disk has been spun and the run-out phenomenon occurs, a pickup head maintains at position 170 and generates a tracking error signal TE which can be used as the source for calibration. When the run-out phenomenon occurs, the physical center of the optical disk swings between the right-shifted position 174 and the left-shifted position 172. When the optical disk shifts to the left side, the stationary pickup head at position 170 is positioned over an inner track 182 of the optical disk. When the optical disk shifts to the right side, the stationary pickup head at position 170 is positioned over an outer track 184, which is the track 194 when the physical center swings to the left-shifted position 172. Thus, when the run-out phenomenon occurs, the stationary pickup head at position 170 has relative motion with the optical disk. The stationary pickup head relatively cross multiple tracks between the tracks 194 and 182, as shown in FIG. 1A, when the optical disk has been rotated, and a sinusoidal tracking error signal, as shown in FIG. 2A, induced by crossing tracks is obtained.
Referring to FIG. 1B, a block diagram of an optical disk drive 100 calibrating a tracking error signal is shown. The optical disk drive 100 comprises a pickup head 102, a sampling module 104, a comparator 106, a calibration module 108, and a compensator 110. During calibration process, the pickup head 102, stationary positioned over the optical disk, first generates a tracking error signal which probably affected by a run-out phenomenon. The sampling module 104 then samples the tracking error signal to obtain characteristic values of the tracking error signal per sampling period. In one embodiment, the characteristic value comprises an average peak-to-peak voltage and an average offset voltage. The comparator 106 then compares the characteristic value with a predetermined target value to determine an adjustment signal. In one embodiment, the adjustment signal comprises a gain adjustment signal and an offset adjustment signal. The calibration module 108 comprises an amplifier 112 and an offset module 114. The amplifier 112 amplifies the tracking error signal according to the gain adjustment signal to obtain a first adjusted tracking error signal TE′ with a desirable amplitude, and the offset module 114 then offsets the first adjusted tracking error signal TE′ to obtain a second adjusted tracking error signal TE″ with a desirable offset level.
During an normal operation, the compensator 110 can then determines a tracking control signal TRO according to the second adjusted tracking error signal TE″ to move the pickup head 102. Thus, movement of the pickup head 102 compensates the swing disturbance of the optical disk to control the pickup head keeping following a track. Referring to FIG. 1C, a wave form of a tracking error signal is shown. The tracking error signal before time t1 swings due to a run-out phenomenon. After a TRO signal is applied to move the pickup head at time t1, the tracking error signal is getting stable.
FIG. 2A shows an example of a tracking error signal obtained when a run-out phenomenon occurs. The tracking error signal comprises two return points 212 and 214. The return points 212 and 214 are the points where phase of the tracking error signal inverts. The return points occur when the physical center of optical disk moves to position 172 or position 174 in FIG. 1A. The periods of the regions 202 and 206 respectively containing the return points 212 and 214 are referred to as “return time”. Each peak between the return points 212 and 214 are caused by a track crossing the pickup head while the optical disk drive swings due to the run-out phenomenon.
An optical disk may be clamped at different positions while tray-in into the optical disk drive; therefore the run-out distance d, as shown in FIG. 1A, accordingly is variable. FIG. 2B shows the distributions of a run-out amount verses the return time of a 100-time tray-in test using the same disk. A run-out amount indicates a number of peaks occurring between return points of the tracking error signal and is proportional to a run-out distance. The test result shows that the run-out amount is variable and has an average of 70 tracks, and the return time is also variable and has an average of 4 ms. It is noted from FIG. 2B, there are still extreme situations, however, with large return times or small run-out amounts, such as points 220˜224. Such extreme situations may cause errors in tracking error signal calibration.
When the run-out shift distance is short such as the points 220˜224 in FIG. 2B with a small run-out amount, the run-out shift distance overlaps only a few tracks, i.e. the run-out amount is small, and only a few peaks are generated in the tracking error signal when the optical disk rotates. For example, the tracking error signal shown in FIG. 2A has a run-out shift distance of about only 5 tracks, because only five peaks occur in the range 204 between the return points 212 and 214. Thus, the frequency of the tracking error signal is very low in this extreme situation. The sampling module 104, however, samples the tracking error signal with the same sampling rate in the described extreme situation to obtain the characteristic values per sampling period. Because the frequency of the tracking error is low, periodically sampled characteristic values may not accurate represent the real characters of the tracking error signal, inducing inappropriate amplification and offset compensation and affecting the calibration of the tracking error signal in the calibration module 108. FIG. 2C shows a calibration result of the tracking error signal when the run-out amount is about 15 tracks. The calibration result shows that the peak-to-peak voltages and offset voltages of the calibrated tracking error signals greatly deviate from the target peak-to-peak voltage of 1.8V and the offset voltage of 1.38V. Thus, a method for adjusting a tracking error signal of an optical disk drive is required.